Similar issues existed for AccountManagerPlugin (#7850), TagsPlugin (#7787)
and others before. Previous code (done by me) was just too inconvenient and
didn't respect current Trac policy to still treat Babel as optional dependency.

Other, not striktly related changes are just more steps towards next release.
This includes especially ongoing documentation of changes.

Refs #4201, Fixes #7856: Version 0.4: Added support for TagsPlugin. Tags from the TagsPlugin will be used whenever the TagsPlugin is installed. In the future, an option might be added to determine whether or not to use the TagsPlugin when it is installed. The changes here were derived from the source as modified by georgesoon, keywordsuggest.py in #4201.

These changes have been lightly tested, so please open a new ticket if you encounter any issues.

Compatibility note: Previously, the separator option was defined in quotes, e.g. ', '. Now, the separator is just defined as a character, without quotes. A space is added to all separators.